I don't know what that is relative to zero. I can tell you with my Onk, I didn't notice a difference on well-recorded material until -2 or -3. At that setting, the M5s weren't exceptionally loud at 14 feet away but I could tell the Onk's compressor was kicking in on peaks. At 0, I could hear distortion. The M100s on the other hand are plenty loud even around -7. As soon as I put the 1500 in, I could drive the M5 to satisfyingly loud levels and that opened up the soundstage and fullness even more. With the 1500 and M100, I can easily melt my ears.

You have good dimensions other than distance to front wall. You should have width and height but little depth in your soundstage. If you can experiment with that front wall distance, you'll be rewarded with amazing depth.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated